Thick phase or volume holograms can be recorded in electro-optic single crystals through diffusion or drift of free carriers which are photo-excited from deep traps. During exposure to a modulated light pattern derived from two intersecting coherent light beams, excess photo-excited electrons are produced in regions of high illumination which migrate from those regions to regions of low illumination and are then trapped by impurity ions in the crystal. For example, holograms can be recorded by generating an interference pattern through interaction between a coherent reference beam and an object beam, the latter carrying spatial modulation corresponding to the image to be recorded. The resulting redistribution of electrons produces a corresponding electric field which causes a change in the index of refraction of the crystal medium. The resultant holographic interference pattern is a stationary, high intensity field produced in the region of overlap between the object beam and the reference beam.
This holographic recording method can achieve high density, noise free recording for various memory and display applications. A more detailed discussion of the recording of phase holograms in single crystal electro-optic materials may be found by referring to the following references: Chen et al., Applied Phys. Letters, Vol. 13, p. 223 (1968); Townsend et al., Journal of Applied Physics, Vol. 11, p. 5188 (1970); and Amodei, Applied Phys. Letters, Vol. 18, p. 22 (1971).
The recorded hologram is read out by coherent light which traverses the crystal medium in the direction of the reference beam and is phase modulated in accordance with the recorded pattern and reproduces the object beam through wavefront reconstruction. During readout of unfixed holograms, the electrons are re-excited out of the traps and redistributed evenly throughout the volume of the crystal, thereby removing the field and erasing the holograms.
In order to obtain improved diffraction efficiency of recorded holograms in electro-optic crystals, the concentration of trapping centers can be increased by increasing the number of impurity ions in the crystal. The sensitivity of the holograms, i.e., the amount of energy required to store a hologram, also depends on the concentration of traps, since they determine the electron generation rate during recording.
Lithium niobate single crystals are known as electro-optic materials useful as holographic recording media which have fairly good sensitivity. The sensitivity, as well as the diffraction efficiency, can be greatly improved by doping such crystals with iron, as has been disclosed in a copending application of Amodei and Phillips, Ser. No. 196,733, filed Nov. 8, 1971, now abandoned. For example, iron-doped lithium niobate crystals have 500 times the recording sensitivity and ten times the diffraction efficiency of undoped crystals.
Iron-doped lithium niobate single crystals can be made by the Czochralski technique whereby approximately equimolar amounts of lithium carbonate and niobium pentoxide are admixed and charged to a platinum crucible. The materials are fused and the desired amount of iron oxide added. The crucible is placed in a resistance heated growing furnace and the melt maintained at about 1260.degree.C. Growth is initiated about a seed crystal, usually c axis oriented. The crystal is pulled from the melt at a rate of about 5mm/hr., preferably while rotating the crystal at 10-30 rpm. When growth is terminated, the crystal is annealed in an isothermal environment at about 1100.degree.C. for 4-5 hours and slowly (50.degree.C. per hour) cooled to room temperature.
Following growth and annealing, the crystals are poled by heating to about 1200.degree.C. in an oxygen-containing atmosphere and passing a small electric current through them, e.g., about 2 ma./cm.sup.2 of cross-sectional area. Since the application of a current tends to reduce the crystals, they are allowed to anneal in oxygen for several hours after poling at a temperature of about 950.degree.-1000.degree.C. and then slowly cooled to room temperature.
For high sensitivity and read-write performance, the erase sensitivity should be as high as the record sensitivity. Normal iron-doped lithium niobate crystals poled and annealed as described above contain iron mainly in the trivalent state and have from three to twenty times lower erase sensitivity than record sensitivity. Thus for optimum erase sensitivity, most of the iron must be reduced to the divalent state. In the past this has been done by heating the doped crystal in an inert gas, such as argon, with a low partial pressure of oxygen. This method is useful when a fraction of the trivalent iron is to be reduced, but when increased reduction is required, the lithium niobate host crystal is reduced as well. The amount of reduction that can be obtained by heating in argon is directly proportional to the heating temperature and is inversely proportional to the total amount of iron present. Heating temperatures are limited to about 1150.degree.C.; heating above that temperature results in depoling of the crystals. For lightly doped crystals, reduction of up to about 90% of the iron present can be achieved, but much less reduction is obtained by this method with increasing iron content or with larger crystals.
Reduction of the lithium niobate crystal itself creates an optical absorption that overlaps the absorption of divalent iron and interferes with the hologram storage. The electrical conductance of the crystal is increased, giving rise to an increased rate of thermal decay of stored holograms, and adversely affecting the lifetime of stored holograms. Thus a new method of reducing the iron in lithium niobate crystals whereby almost all of the iron is reduced to the divalent state without concomitantly reducing the lithium niobate is highly desirable for rapid erasure of information stored in these crystals.